The periodic pitch of current coaxial helicopter is realized by controlling the lower blade pitch with swashplate under the control of steering engine. To realize control over blade pitch of upper blade under the control of steering engine, it needs to firstly control lower swashplate, and then transfers motion to upper swashplate via link mechanism. Its weakness is: the transmission distance of pitch motion under the control of steering engine is long, involving more mechanisms, big accumulated error of fit clearances among many links, poor mechanism rigidity, and thus affecting the pitch control precision and control stability. The current pitch control of upper blade in the control of differential course of coaxial helicopter is realized by controlling connecting rod with steering engine, whereas the connecting rod can only control blade pitch of upper blade through the whole center shaft. Its weakness is: the transmission distance of motion is long; the structure is complicated and the manufacturing of center shaft is difficult; and thus it is not suitable for using in small-sized helicopters.